[cairo] Spot colors (and CMYK)

[ecir hana]

On Tue, Feb 16, 2010 at 11:49 PM, Bill Spitzak <spitzak at gmail.com> wrote:
> Kai-Uwe Behrmann wrote:
>
>>> However we all agree that unclamped sRGB values can be converted to
>>> all other 3-dimensional color spaces, including XYZ and Lab. So it is
>>> sufficient for all 3D spaces.
>>
>> The CMS Oyranos uses end to end colour transforms bypassing the
>> 3-dimansional PCS during colour conversion.
>
> Bypassing is not the same. If I have a transform from 4D to 3D and
> another transform from 3D to 4D, I certainly can multiply them into a
> single 4D->4D transform. That does not cause more than 3D of data to be
> passed, the same points that map together still will. That would be like
> claiming that if you multiply the conversion from Dollars to Pounds and
> Pounds to Euro, you can convert monetary values that cannot be specified
> in Pounds.
>
> ICC describes all devices in a 3D space. Therefore any color management
> based on it can be controlled fully by a 3D color description. However I
> am open to proof of a popular device description that uses more than 3
> dimensions to describe the intermediate color form. But I have not seen
> anything other than "spot colors" which by definition CANNOT be
> transformed to other devices (a 3D simulation can but is
> indistinguishable from that same 3D simulation specified without the
> spot color).

Now this is interesting. If I go from CMYK A to CMYK B:

- Kai-Uwe Behrmann, does Oyranos really skips LAB? How does it know
what value CMYK A (40, 30, 30, 100) should be in CMYK B? Is this what
Chris Murphy and Jon Cruz call "device-link" ICC and "smaller delta"?
http://lists.cairographics.org/archives/cairo/2010-February/019130.html
http://lists.cairographics.org/archives/cairo/2010-February/019128.html
http://lists.cairographics.org/archives/cairo/2010-February/019101.html

- Bill Spitzak, are you saying that I always have to go trough 3D
space? You know that 4D->3D + 3D -> 4D is not the same as straight 4D
-> 4D. Are you saying that even if I could combine "4D -> 3D" and "3D
-> 4D" into one operation (for performance reasons?) it still
effectively remains 3D bounded?